1. Technical Field
The present invention relates in general to graphics display systems, and more particularly to a method and system for independent control of multiple windows in a graphics display system. Still more particularly, the present invention relates to a method and system for independent control of overlay and color planes in a graphics display system.
2. Description of the Related Art
Computer graphics display systems of contemporary design use windows to highlight or concurrently display independent blocks of information. The user of the system routinely has the power to operate within a window, operate in areas outside a window, or to relate activities of various windows.
U.S. Pat. No. 4,317,114, Walker, entitled Composite Display Device For Combining Image Data and Method, issued Feb. 23, 1982, teaches a method for displaying a set of overlay images superimposed over a host image on a display monitor. U.S. Pat. No. 4,954,818, Nakane et al., entitled Multi-Window Display Control System, issued Sep. 4, 1990, discloses a method and system for providing a multi-window display control system where a plurality of data are overlapped and displayed simultaneously on one display monitor.
As known in the art, the image displayed on the display monitor is typically stored in a memory array known as a frame buffer. The frame buffer is periodically scanned or otherwise accessed to ascertain the color, intensity and other information used to generate the image on the display monitor itself. A frame buffer contains, among other items, color planes, overlay planes and window identification planes.
Color planes are comprised of pixels and contain a rendered image for display on the display monitor. Overlay planes, also comprised of pixels, are used to supersede the pixels in the color planes. The image in the color planes remains intact while the portion of the image in the overlay planes can be moved independently of the total image, and can be superimposed over at least a portion of the image in the color planes.
As discussed above, windows are independent portions of the screen that each represent an application. The image as stored in the frame buffer normally includes the resultant effect of overlying windows because each window is assigned an identifier number that is drawn into the window identification planes to define the boundaries of the window. As each application creates an image or "renders" to the color planes, the window identifier associated with the application defines the area where the application can be drawn. In other words, only the portion of the image which will be displayed in the window is sent to the color planes.
Contemporary graphics display systems typically contain only one set of window identification planes. As a result, overlay planes are constrained to the same window boundaries as the color planes for any particular window identifier. Therefore, when an image in the overlay planes is superimposed over at least a portion of the image in a color planes, the color data can not be rendered to the pixels in the color planes which lie underneath the image in the overlay planes. Consequently, when a window is removed from view, the image in the color planes must be regenerated in the changed region of the frame buffer.
Another problem encountered with the utilization of one set of window identification planes involves transparent areas within an overlay. A user may want to have all or a portion of an image in the overlay plane be transparent so that the color image below the image in the overlay plane is displayed. This is not possible with systems which utilize only one set of window identification planes. The window identifier in the area of the overlay can be utilized only by the overlay image. A window identifier for the image below the overlay image is not available. Thus, the transparency effect can not be achieved in such systems because proper pixel interpretation is impossible.
Therefore, it should be obvious that a need exists for a system and method which provides for independent control of color and overlay planes in a graphics display system.